Inverters change one type of electrical current into another. There exist two types of electrical current, direct current (DC) and alternating current (AC). The electricity commonly available in mobile situations via batteries or generated via alternative means, e.g., wind generators or solar panels, is DC. DC can be easily stored using well known means such as batteries or capacitors. To be used with common appliances and other wall-powered devices, DC must be converted to AC.
Direct current is current which flows in the same direction at all points in time. If one were to measure the voltage of a DC circuit at different instants in time, the measurement would remain constant. As mentioned, the advantage of DC is that it is easy to store.
Alternating current is current which periodically reverses its direction of movement over various periods of time. If one were to measure the voltage of an AC circuit at different instants in time, the voltage would fluctuate, being in a cycle of continuous reversal. In the U.S. this cycling occurs 60 times per second, i.e., 60 Hz. The advantages of AC are that it is very easy to step voltages up or down (through transformers) and thus easier to distribute over long distances with smaller wire than would be possible with DC. This is because as electricity is carried, energy is thermally dissipated due to the resistance of the wire. However, the relative loss decreases as the voltage increases.
As mentioned previously, DC must be converted into AC to power appliances and other wall powered devices. This is the role inverters play. Many methods of DC to AC conversion are well known in the art. However, they all present serious shortcomings that the present invention addresses in a novel fashion.
One method known in the art for DC to AC conversion and DC to DC conversion is voltage controlled pulse width modulation. High frequency switched DC to AC inverters generally use a voltage controlled pulse width modulation scheme such as the system 100 exemplified in FIG. 1 (FIG. 1) wherein DC current enters at terminals 108 and AC current leaves at terminals 109. This system has a full bridge configuration of switching transistors and commutating diodes 106. Said bridge could, for example, comprise transistors of type Bipolar, IGBT (insulated gate bipolar transistor), MOSFET (metal-oxide semiconductor field effect transistor), or gate controlled SCR (silicon controlled rectifier). Said bridge is then connected to an LC (inductor and capacitor) output filter 107. The semiconductors are enabled by conventional drive circuitry 105. The circuit operates by pulse width modulating a constant frequency drive to the switching transistors in such a way that the average output from them, when smoothed by the LC filter 107, is the required low frequency sine wave.
A sawtooth generator 102 provides a constant frequency constant amplitude sawtooth ramp signal derived from a conventional relaxation oscillator operating at the required high switching frequency. A low voltage reference sine wave is generated by 101 by conventional means and has a peak to peak amplitude slightly less than that of the sawtooth ramp. In the case of a DC to DC converter the sine wave reference is replaced by a DC voltage reference.
The sine wave or DC voltage reference and sawtooth reference are then compared by a conventional analog comparator 104 which acts here as a pulse width modulator to generate a pulse width modulated logic level signal which if passed through a low pass filter will accurately reproduce the sine wave or DC reference. The modulated signal is then buffered and isolated by the transistor drive circuits 105 for connection to the bridge power switching transistors and commutating diodes 106. An LC filter 107 removes high frequency components to leave a low frequency sinusoidal or DC voltage output.
However, line and load regulation are quite poor with this type of circuit. One method to improve the regulation is shown with the addition of an output meter 103 which produces a DC error signal to control the sine wave reference output voltage. Such control is by its very nature slow and reacts poorly to switched and non linear loads. Other output correction schemes have an error amplifier connected in the same way as for a DC to DC converter but in this case the phase shift caused by the LC output filter 107 is considerable, even at the low output frequency, and it is hard or impossible to achieve the high loop gain that is necessary for good performance when the inverter or DC to DC converter drives non-linear or pulsed loads.
Another method well known in the art for DC to AC or DC to DC conversion is current mode with pulse width modulation. FIG. 2 (FIG. 2) shows a modification of the voltage controlled pulse width modulated system 200 to allow current mode control wherein DC current enters at terminals 108 and AC or DC current leaves at terminals 109. A current sense point 201 is inserted between the switching power transistors 106 and the LC output filter 107 to provide a reference voltage proportional to the instantaneous current. In this system the inverter or DC to DC converter output voltage at terminals 109 is compared to a reference sine wave by an error amplifier 202. The intention is to make the current flowing through the power switches 106 proportional to this error voltage and as a consequence the power stage becomes a high impedance current source; the output inductor impedance is absorbed into the high impedance source and thus the maximum phase shift through said filter 107 is now only 90 degrees compared to 180 degrees for a voltage control system.
The error voltage from the error amplifier, or voltage comparator 202 as it is often referred to in a current controlled system, is compared with the current reference signal in the current comparator 203 to produce a current error signal. This signal is now compared with a high frequency sawtooth reference by comparator/pulse width modulator 104 and the high frequency digital output is connected to the transistor drive circuits 105 as in the above disclosed voltage controlled pulse width modulated inverter 100.
The resulting system provides true current mode control but unfortunately inherits the enormous disadvantage of an inherent form of instability known as "subharmonic oscillation" that is prevalent in current mode systems for which the duty cycle is either more or less than 50% depending on the configuration. As an inverter requires pulse widths between 0% and 100% of the duty cycle the problem is unavoidable with this type of control.
In practice the effects of subharmonic oscillation do not become significant until the output filter inductor is made small and the high frequency components of the inductor current exceed 5% of the maximum current. This restriction makes the system unsuitable for very small, lightweight inverters.
Another method known in the art for DC to AC inverters is hysteretic current control. High performance high frequency switching inverters and DC to DC converters require gain around the control loop at frequencies many multiples of the baseband sine wave. This is particularly true in the case of inverters driving non-linear loads such as diode rectifiers with capacitor filters for which a high loop gain at frequencies greater than ten or twenty times the baseband frequency is essential if the waveform distortion is to be minimized. Hysteretic current control achieves such performance without becoming prone to subharmonic oscillations. Unfortunately it does not work well at switching frequencies above 50 kHz where circuit delays and power component switching times become so long that circuit currents change significantly between the time that a specific current level is measured and the actual change of state in the power circuits.
An alternate solution to hysteretic current mode control has been achieved via the present invention by adapting the current mode control inverter with pulse width modulation in such a way that eliminates the possibility of subharmonic oscillations. This is achieved by a pulse width modulation scheme that is not constrained in time.
The systems previously described, both prior art and what has to this point been disclosed, have featured two state modulation. FIG. 3A shows the operation of a two state bridge power stage 400 in detail. Q1, Q2, Q3, and Q4 are the switching elements and L and C form the output low pass filter 107. The drive circuits are connected so that at one time Q1 and Q4 are switched on with Q2 and Q3 switched off. In the other switching phase, Q1 and Q4 are switched off with Q2 and Q3 switched on. Thus, referring to FIG. 3B, the voltage at D mirrors that at C and the switching losses are identical for both bridge halves, Q1, Q2 and Q3, Q4. These losses are considerable and can be minimized if one side of the bridge Q1, Q2 switches at the low output frequency while the other side Q3, Q4 switches at the high switching frequency. This is known as "three state" switching. An embodiment of the present invention will be disclosed utilizing three state switching as a means to higher efficiency.
All of the inverter circuits disclosed have yet another problem addressed by a further embodiment of the present invention. When an AC generator is brought on line or incurs an overload its output voltage falls but retains its sinusoidal quality, although the frequency may change. DC to AC inverters that are controlled by a sine wave reference signal, however, retain their frequency but suffer from a clipping of the tops and bottoms of their waveforms. Clipped waveforms are undesirable because they lead to unacceptably high levels of harmonics of the baseband frequency that can cause overheating in electrical machines and also high frequency emissions. To address these concerns, a sine wave compression circuit will be disclosed.
In view of the foregoing, clearly there exists a need for an improved power inverter and DC to DC converter that addresses the shortcomings of the prior art, e.g., poor regulation, instability, and inefficiency.